The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a printer, a facsimile, or a multifunction apparatus formed by combining them, and in particular, to an improvement in a cleaning technique for an image forming apparatus.
Up to now, image forming apparatuses utilizing the electrophotographic system (the electrostatic transfer system), such as copying machines and printers, are widely known. In such image forming apparatuses, a toner image is transferred to a recording sheet by a final transfer member, and then the toner image is fixed to thereby obtain a toner image as a permanent image on the recording sheet. Here, it is necessary to remove the toner which has not been transferred to the recording sheet by means of a cleaning device in the image forming apparatus. Apart from this ordinary image formation, a so-called process control is also performed, in which reference marks are formed on the image bearing member, the intermediate transfer member, and the final transfer member, and in which registration control and toner image density control are effected on the basis of the positions, density, etc. of the reference marks (patches). Here, in the process control, the reference marks (patches) are not usually transferred to the recording sheet, so that the marks have to be removed by the cleaning device in the image forming apparatus.
Regarding the cleaning inside the image forming apparatus, the following techniques have been proposed. JP 2001-75448 A discloses a technique in which the residual toner on the intermediate transfer member and the final transfer member is collected on the final transfer member, and collectively removed by a cleaning device which is in contact with the final transfer member. Further, there might be a technique according to which the final transfer member is spaced apart from the intermediate transfer member during the process control so that the reference marks may not adhere to the final transfer member.
However, in many cases, the final transfer member is arranged at an end portion inside the image forming apparatus, and the construction in which there is further provided a cleaning device, which is in contact with the final transfer member, is likely to lead to an increase in the size of the image forming apparatus. Further, when the final transfer member is constructed so as to allow contact and separation to and from the intermediate transfer member, the apparatus is rather complicated, resulting in an increase in cost.
The present invention has been made in view of the above-mentioned technical problem, and provides an image forming apparatus which is compact and inexpensive, and which allows cleaning of the final transfer member.
Thus, according to an aspect of the present invention, an image forming apparatus includes: an image bearing member; an image forming unit that forms a toner image on the surface of the image bearing member; an intermediate transfer device which is formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit that controls the potential gradient between the image bearing member, the intermediate transfer member, the final transfer member, and the cleaning member, in which the control unit forms in an image formation mode a potential gradient for image formation which causes reverse-polarity toner on the final transfer member to adhere to the cleaning member by way of the intermediate transfer member.
Here, the image forming units include a charging unit that uniformly charges the surface of the image bearing member, a latent image forming unit (exposure unit) that forms an electrostatic latent image corresponding to the output image on the surface of the image bearing member, and a developing unit that selectively imparts toner to the electrostatic latent image formed on the surface of the image bearing member. A variation of the charging unit is a contact type charging unit which is in contact with the image bearing member and is adapted to uniformly charge the surface thereof. In this case, the control unit is capable of controlling the potential gradient not only between the image bearing member, the intermediate transfer member, the final transfer member, and the cleaning member, but also between them and the contact type charging unit.
By thus constructing the image forming apparatus, the reverse-polarity toner on the final transfer member adheres to the cleaning member by way of the intermediate transfer member in the image formation mode, so that, firstly, there is no need to provide the final transfer member with a cleaning device. That is, it is possible to construct the image forming apparatus such that the cleaning member does not come into contact with (or is held in a non-contact state with respect to) the final transfer member. Secondly, there is no need to frequently perform the cleaning mode operation. The image forming potential gradient is a potential gradient which allows the toner image as the output image on the image bearing member to be transferred to the recording sheet transported between the intermediate transfer member and the final transfer member by way of the intermediate transfer member.
According to another aspect of the present invention, the image forming apparatus includes a detecting unit that detects the density of the toner image on the intermediate transfer member, in which the control unit forms in a process control mode a potential gradient for process control which causes the toner image as a detection image on the image bearing member to adhere to the cleaning member by way of the intermediate transfer member (without adhering to the final transfer member). In this case, the process control mode performs registration control and toner image density control.
By thus constructing the image forming apparatus, it is possible to prevent the toner image as the detection image from adhering to the final transfer member, without providing a retracting mechanism for making the final transfer member abut against and space apart from the intermediate transfer member. As a result, no problem is involved even if the final transfer member is constantly held in contact with the intermediate transfer member (constantly held in contact therewith also in the process control mode, the image formation mode, and the cleaning mode except when the image forming unit is replaced).
Here, the difference in potential between the final transfer member and the intermediate transfer member which is in contact with the final transfer member when forming the potential gradient for the process control may range from 0 to 1500 [V]. When this difference in potential is ensured, there is little variation in the surface potential of the intermediate transfer member, and it is possible to detect the density of the toner image on the intermediate transfer member in a stable manner.
Further, comparison of the potential gradient for image formation and the potential gradient for process control shows that the potential relationship (potential gradient) between the intermediate transfer member and the final transfer member is reversed between the potential gradient for image formation and the potential gradient for process control. Further, the potential relationship (potential gradient) between the intermediate transfer member and the cleaning member is reversed between the potential gradient for image formation and the potential gradient for process control.
Further, the control unit effects transition of the state of the image forming apparatus from the process control mode to the image formation mode. More specifically, the control unit effects transition of the state of the image forming apparatus from the process control mode directly to the image formation mode without passing the stage of the cleaning mode. Further, it is also possible to construct the control unit such that selection is appropriately made between the case in which transition of the state of the image forming apparatus is effected from the process control mode directly to the image formation mode without passing the stage of the cleaning mode and the case in which transition is effected from the process control mode to the image formation mode through the cleaning mode.
The operation of the process control mode may be performed immediately after turning on the power of the image forming apparatus or between one image formation mode and the next image formation mode.
While the image forming apparatus of the present invention may be formed as a monochrome image forming apparatus, it may also be formed as a multicolor image forming apparatus. When it is formed as a multicolor image forming apparatus, it is possible to adopt a construction which has as the image bearing member plural image bearing members for different colors and which has as the intermediate transfer device a single intermediate transfer member, or a construction which has as the image bearing member plural image bearing members for different colors and which has as the intermediate transfer device a first upstream side intermediate transfer member which is in contact with a part of the plural image bearing members, a first downstream side intermediate transfer member which is in contact with a part of the remaining ones of the plural image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member, the final transfer member being in contact with the second intermediate transfer member. As the plural image bearing members, it is possible to provide three image bearing members for yellow, magenta, and cyan, or four image bearing members for yellow, magenta, cyan, and black. Further, it is also possible to provide image bearing members for other colors.
In the case of providing as the image bearing member four image bearing members for yellow, magenta, cyan, and black, there are provided as the intermediate transfer device a first upstream side intermediate transfer member and a first downstream side intermediate transfer member which are respectively in contact with two of the four image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member, in which the final transfer member is in contact with the second intermediate transfer member.
When the intermediate transfer device is thus formed by the first (upstream side and downstream side) intermediate transfer members and the second intermediate transfer member, the detecting unit can detect the density of the toner images on the first (upstream side and downstream side) intermediate transfer members and the density of the toner image on the second intermediate transfer member.
Further, as the cleaning member, it is also possible to provide a first upstream side cleaning member which is in contact with the first upstream side intermediate transfer member and a second cleaning member which is in contact with the second intermediate transfer member. In this case, the control unit can form in the image formation mode a potential gradient for image formation in which the reverse-polarity toner on the final transfer member adheres to the second cleaning member by way of the second intermediate transfer member, and can form in the process control mode a potential gradient for process control in which the toner image as the detection image on the image bearing member adheres to the second cleaning member by way of the first upstream side intermediate transfer member, the first downstream side intermediate transfer member, and the second intermediate transfer member (without adhering to the final transfer member). Further, it can form in the process control mode a potential gradient for process control in which the reverse-polarity toner on the final transfer member adheres to the first upstream side cleaning member by way of the second intermediate transfer member and the first upstream side intermediate transfer member.
Further, as the cleaning member, it is also possible to provide a first downstream side cleaning member which is in contact with the first downstream side intermediate transfer member. In this case, the control unit can form in the process control mode a potential gradient for process control in which the reverse-polarity toner on the final transfer member adheres to the first downstream side cleaning member by way of the second intermediate transfer member and the first downstream side intermediate transfer member.
Further, the cleaning member is a cleaning roll and is provided with a cleaning device having a cleaning blade in press contact with the cleaning roll and an accommodating member which recovers toner removed through cleaning by the cleaning roll and the cleaning blade.
Further, the surface roughness (Rz) of the final transfer member may be 20 [xcexcm (Rz)] or less, and the surface roughness (Rz) of the intermediate transfer member may be 10 [xcexcm (Rz)] or less. Furthermore, the final transfer member may have a higher degree of surface roughness (Rz) than the intermediate transfer member. It is desirable for these surface roughnesses (Rz) to be not more than the average grain size of the toner forming the toner image. When these conditions are satisfied, it is advantageously easy for the residual toner to move from the final transfer member to the intermediate transfer member (the second intermediate transfer member). The surface of the final transfer member may be formed of a resin coating layer or a resin tube layer.
In accordance with the present invention, it is possible to provide an image forming apparatus which is compact and inexpensive and which allows cleaning of the final transfer member.